Presently available FM modulators are embodied in essentially analog hardware, including either a VCO or a VCXO.
A significant problem with the VCO-implemented FM modulators is frequency instability due, at least in part, to sensitivity to drifts and/or inaccuracies in operating voltage and manufacturing tolerances. A further problem with VCO-implemented FM modulators is the presence of noise in the FM output signal thereof, which critically limits the accuracy and resolution thereof.
The utilization of a VCXO in place of a VCO minimizes the above-noted problems experienced with VCO-implemented FM modulators, but not without the serious drawback of additional hardware requirements.
Due to greatly increased traffic handling capability requirements, digital cellular radiotelephone systems are currently being developed to supplement the existing analog cellular radiotelephone systems. However, in order to accommodate communications between the soon-to-be installed digital components and the already installed analog components of the resultant hybrid cellular infrastructure, it will be necessary for the individual mobile subscriber units (i.e. the hand-held and auto-installed cellular telephone sets) to have the versatility to interface with base stations operating in either the digital or the analog domain. These so-called dual-mode subscriber units must therefore have the capability to FM modulate (and discriminate) voice/data signals in either domain.
Since the dual-mode subscriber units are consumer items, cost, performance and size are highly sensitive parameters with regard to the marketing thereof. Currently available analog FM modulators, as discussed above, suffer from significant shortcomings and disadvantages which limit their utility in dual-mode subscriber units, since the utilization of these currently available analog FM modulators in this application would result in degraded performance and/or increased cost and size of the dual-mode subscriber units, relative to dual-mode subscriber units which utilize the essentially digital FM modulator of the present invention, which overcomes the limitations and drawbacks of these currently available analog FM modulators, for reasons which will become apparent in the ensuing brief and detailed descriptions of the present invention.